grit, which one

Do chickens have to have a mixture of oyster shell and normal grit or can they just do with oyster shell alone, ive just been trying to find some and have been quoted these, can you tell me if there just too expensive

Oyster shell    25kg  £7.50
Fine Grit          25kg  £8.00

carnt seem to find anywhere else that does it without slapping on expensive shipping fees.             nicky